1. Field of the Invention
The present invention relates to an automatic gain control apparatus for a motor servo system.
2. Description of the Prior Art
In FIG. 1 illustrating a block diagram of a conventional motor speed control circuit, 1 designates a frequency-voltage converter; 2 an error voltage amplifier; 3 a motor; 4 a motor speed detector for producing a sinusoidal or pulsate signal proportional to the number of revolution or a motor speed; 5 a reference power source; 17 an amplifier; 19 a pulse generator. In FIG. 1, the motor speed detector fetches a signal with a frequency corresponding to the motor speed of the motor 3. The fetched signal is converted into a DC voltage by the frequency-voltage converter 1. The converted DC voltage is compared with a reference voltage of the reference power source 5 by the error voltage amplifier 2. By controlling the drive voltage of the motor 3 by a voltage in accordance with an error, the motor speed control circuit operates so as to minimize the difference between the voltages of the reference power source and the frequency-voltage converter 1. For setting the motor speed to a desired value, it may be selected to change the gain of the frequency-voltage converter 1 or to change the voltage of the reference power source 5. It is a common practice, however, that the gain of the frequency-voltage converter as shown in FIG. 2 is changed for this purpose. It is assumed that to an input terminal 6 is applied a signal proportional to the motor speed, namely, a pulse as shown in FIG. 3(a). This pulse drives a reset switch 12, a pulse shown in FIG. 3(b) delayed by a given amount of delay .tau. by a delay circuit 7 drives a sampling switch 11, a pulse as shown in FIG. 3(c) delayed by the given amount of delay .tau. by another delay circuit 8 drives a reset switch 10. With such a construction, a capacitor 13 is charged by a constant current Io from a constant current source 9 for a given period of time and is instantaneously discharged by the reset switch 10. The cycle of the charge and the instantaneous discharge to and from the capacitor is repeated to produce a signal with a triangular waveform as shown in FIG. 3(d). In order to sample-and-hold the peak value of the triangular wave in a capacitor 14, the sampling switch 11 is operated by the output from the delay circuit 7 immediately before the reset switch 10 is operated. The voltage across the capacitor 14 must be reset for each period to obtain the hold voltage which is always proportional to the motor speed in accordance with a change of the motor speed. To this end, the pulse shown in FIG. 3(a) is used to reset a reset switch 12. Through the operation as mentioned above, the waveform of the voltage across the capacitor 14 becomes as shown in FIG. 3(e). If the pulse width of each pulse (a) to (c) in FIG. 3 is satisfactorily shorter than a repetitive period T, a voltage substantially equal to the peak value shown in FIG. 3(d) may be obtained at the output terminal 16 by making the voltage across the hold capacitor pass through a low-pass filter 15.
The peak voltage V across the capacitor 13 is given by the following equation EQU V=Io/C.times.T=Io/C.times.1/f (1)
where Io is the current from the constant current source 9, C is a capacitance of the capacitor 13, T is a repetitive period, and f is the frequency. The circuit shown in FIG. 1 operates so that the peak voltage V is approximately equal to the reference voltage Vr across the reference voltage source 5. In this circuit, a set period Tr may be changed by changing the reference voltage Vr or the constant current Io. In this case, it is preferable to use the constant current Io for changing the set period Tr. The reason for this is that a changeable range of the voltage Vr is restrictive for the reason of the limited power source voltage. From the equation (1), the gain K of the frequency-voltage converter is given EQU K=dV/df=-Io/C.times.1/f.sup.2 ( 2)
Hence, with respect to the gain K1 at the frequency f.sub.o, a gain Kn at a frequency nf.sub.o obtained when the current Io is increased by n times, is expressed by EQU Kn=dv/dt.sub.f=nf.sbsb.o =-NIo/C.times.1/(nf.sub.o).sup.2 =Kl/n (3)
As seen from the above equation, the gain becomes small proportionally to the motor speed. In other words, when the motor speed is small, the gain is large, while when it is large, the gain is small. In the usual motor servo system, where there is a single set frequency, the above-mentioned phenomena does not give rise to any problem. In the case where an apparatus has many set frequencies, such as a video tape recorder in which a capstan motor speed is set to 1/n, double, constant, or n times speeds, the gain must be substantially constant by changing the gain of the error voltage amplifier; otherwise, the servo system tends toward oscillation or provides insufficient system drive, failing to obtain a desired control characteristic. Most of the conventional motors have been designed to have a single set frequency. Therefore, all a designer has to do is to merely change the circuit constant of the error voltage amplifier in accordance with the kind of the motor used. In the case where a number of set motor speeds, i.e. set frequencies, are assigned to a single motor within a circuit, it is difficult to change the gain of the error voltage amplifier 2 by merely changing the constant of the reference power source 5 for each frequency. When the circuits shown in FIGS. 1 and 2 are fabricated by an IC technology, it is impossible to change the circuit constants in the IC circuits fabricated. For determining the gain in such IC circuits, many additional components are attached to the IC substrate, together with their associated circuits to switch the components. This makes the circuit construction complicated. In this respect, it is uneconomical.